Wetting and dispersion of pigments



atented July 6, 1943 aszas'zi warms AND DISPERSION or PIGMENTS Forrest L. Turbett, Joplin, Mo., assignor to The Eagle-Picher Lead Company, Cincinnati, 011i), a corporation of Ohio No Drawing Application June a, 1941,

. Serial No. 396,433

3 Claims.

My invention relates to the wetting and dispersion of pigments, and more in particular it relates to an improved method of manufacturing paints, printing inks and lacquers, and the product made thereby.

Modern pigments have very fine particle size. The large surface energy of the particles causes aggregation and finally agglomeration and the object of modern paint grinding is to break down these agglomerates and aggregates into individual particles and to wet and disperse them throughout the paint vehicle. The greater the degree of dispersion of the pigment in the vehicle the more emcient is the paint in such matters as hiding power, gloss, smoothness of texture, brushability, tint homogeneity, tint retentlon, Weathering power and pigment suspending properties.

In the present day practices of paint grinding, dispersion is accomplished in three known ways.

1. The pressure and friction method in which the paint is ground by means of the steel roller mill or buhrstone mill.

2. The disperser mixing method in which the pigment is dispersed in a special mixer, such as the Banbury mixer.

3. The ball or rod mill method.

In the first instance the partially dispersed mixtures of pigment agglomerates and vehicle in the form of a paste are forced between two or more closely running hard surfaces, such as in the modern steel roller mill or the older buhrstone mill. In the second instance the dispersion is accomplished in a special type of mixer, such as the Banbury mixer, the Werner-Pfleiderer mixer or the Baker-Perkins mixer, which uses exceedingly viscous pastes and depends entire y upon compression to accomplish dispersion. Since in these two methods of paint grinding the successful dispersion of th pigment in the vehicle is to large extent dependent upon shearing forces or internal stresses to accomplish dispersion, it is essential that the pigment-oil paste have rather high viscosity, and be in a more or less pasty condition as it passes through the mixer or mill. Because of the necessity of employing high viscosity mixtures in the initial quality at all times.

accomplished by means of a combination of forces such a that of pressure, shearing action and internal stress existing in the moving mass of balls or rods and paint. The viscosity of the mixture in the ball or rod mill operation is somewhat lower than in the roller mill, buhrstone, or disperser mixer operation, and the pressure, shear and internal stress within. the mass is a great deal less. Therefore, in order to accomplish dispersion it is necessary to process the paint batch for prolonged periods of time, extending to as long as 96 hours per batch, where very good dispersion is desired.

In each of the processes used the vehicle is always in the continuous phase. In the roller mill or buhrstone, a rough mix of the pigment agglomerates is stirred mm the vehicle and passed between the grinding surfaces so that regardless of the state of comminution of the pigment, the vehicle is always surrounding it in a continuous phase. In the pebble or tube mill, the pigment and vehicle may be introduced as separate products, but after the first few turns of the mill, the vehicle always surrounds the pigment particles in exactly the same manner. While the Banbury or Werner-Pficiderer mixer pulls, the pigment agglomerate apart rather than crushes it, the vehicle always surrounds the pigment particles regardless of their size.

It is well known that in these processes there is a problem of producing a product of uniform This is caused by certain inherent faults in the construction of the machinery which make it impossible to adjust and maintain the adjustment of a machine at fixed clearances throughout the operating period. A

' further objection is, that there is no known single machine that will accomplish the grinding of all types of paint in the most satisfactory and,

economical manner, audit is, therefore, a common circumstance to find all three of the known paint manufacturing methods in use in a single plant.

In analyzing the factors involved in the dispei-sion of pigments in vehicles it appeared to me that all these processes involve an unusually large waste of energy caused in part by high frictional or non-useful loads entailed in the starting and maintaining the movement of heavy machines having in most instances, low clearance areas between massive, slow moving parts. In addition, other losses are involved in maintaining the continuous movement of large dead weights of heavy viscous materials in process. The loss of energy caused by this drag of dead weight plus the frictional losses inherent in the machine itself is, inmost instancesQout' of proportion to the useful energy consumed in the actual grinding process. I V

According to my invention, the coarse pigment agglomerates are broken up into much finer particles. At the same time the vehicle is subdivided in the same manner. The inherent physical adhesion or attraction between the fine pigment particles of high surface energy and the fine droplets of vehicle formed at the same time causes the pigment particles which are freed from vehicle during the shattering process to be completely coated by vehicle immediately afterward. More specifically, my process comprises bringing the pigment and vehicle in proportions calculated to form a complete paint into contact with an impactor which reduced them to a fine state of subdivision, suspending the disintegrated particles in a gaseous medium, which hereafter will be referred to in the specification and claims as a state of aeriform suspension, simultaneously bringing the suspended pigment particles into contact with the disintegrated particles of the suspended vehicle, surrounding the suspendedparticles in an envelope of the suspended vehicle to wet the particles with the vehicle and coalescing the pigment and vehicle from the dispersed state to form the finished paint. The impact may be applied in any convenient manner, as for example by means of a moving column or jet of high pressure air or other gas.

The invention may be carried out through the adaptation of the simplest type of equipment.

One suitable form of apparatus is a modification.

of the ordinary hammer mill of which there are many types available. Another form of apparatus which may operated paint spray gun.

In order to illustrate the practice of my invention I will give examples thereof to indicate the means and methods whereby the same may be accomplished and where I have shown a preferred embodiment thereof, it is to be understood the same is subject of modification and change and contemplates other means and methods and substitution of parts, it being my intention to include all means and methods for accomplishing the results desired without departing from the spirit of the invention described and claimed.

EXAMPLE I In this example a white house paint was made. I used an 8 inch diameter rotary hammer mill driven by a 3 H. P. motor. A fiat steel baflie plate was placed at an angle in a receiving vessel below the mill in order to form a coalescing surface against which any fine mist or droplets of paint coming from the mill may impinge. The follow- The pigments were mixed with the vehicle in a change can mixer to form a rough or partially wetted pigment suspension. The suspension was passed through the rotary hammer mill together with dispersing medium, in this instance air, with the rotary impactors moving at a peripheral speed of 19,000 feet per minute. The

ascaev'r feed was controlled at a rate just sumcient to cause the 3 H. P. motor to draw exactly 8 amperes of current at 230 volts, or approximately 1.84 k. w. Under these conditions the mill produced a well dispersed paint at a constant rate of 22 gallons per hour with a power consumption of .081 k. w. h. per gallon, with the minimum of attention. I

In this example the pigment and vehicle upon contacting the rapidly moving hammers are repeatedly struck with an impact of such force as to reduce them to a high degree of subdivision, in a state of aeriform suspension. While in this state the finely divided pigment particles came into contact with the finely divided oil particles which formed an oil envelope around the pigment particles, to completely wet them. The

be used is the well known air centrifugal force set up by the rapidly rotating hammers threw the suspension against the inner surface of the peripheral incasement which broke the suspension, the gas escaping; the pigment and vehicle coalescing to form paint. A certain portion of the paint in this instance came through the discharge screen in a state of fine mist or tiny paint droplets which promptly coalesced upon striking the steel baiiie plate in the container beneath the mill.

For comparison, two other paints having the same composition as the above were made by means of a 3 roll roller mill, having rolls 4 inches in diameter and 8 inches in length and powered upon examination, was found to contain a certain portion of small undispersed pigment a glomerates. Therefore, in order to obtain a product having a degree of dispersion comparable to the impact dispersed product. about one-half of it was put through the mill 9. second time, the remaining one-half beingretained for mixing into paint. The two pastes were then mixed with the proper quantity of oil and drier to form paints of exactly the same composition as the impact dispersed product. I

From the grinding data. obtained, I found the power consumed in producing the single milled product, calculated in terms of finished paint, to be .202 k. w. h. per gallon. For the double milled product the power consumed was approximately double, or .404 k. w. h. per gallon.

Relative hiding power, weight per gallon and degree of dispersion were then accurately determined on the three paints and the results are given in the table below:

' The hiding power test was made bythe checker board b'rushout method in which the paint is brushed out on a known area of special cardboard having black and white squares of checker board design. The relative hiding power of the paints required to completely obscure the black areas of the background.

Dispersion rating was obtained through a modification of Method 1 found on page 197 of the 9th edition of Gardners Physical and Chemical Examination of Paints, varnishes, Lacquers and Colors, in which small portions of the paints were thinned with equal volumes of turpentine, and poured on to a clean glass plate, after which the plate was held in an inclined position in suchmanner that the thin paints spread and ran down the plate. In this test any undispersed pigment agglomerates in the paints could be plainly seen in the thin semi-transparent paint, film.

Weight per gallon was obtained by means of the standard weight per gallon cup and a sensitive balance. The lower weight per gallon of the impact dispersed product indicated the presence of occluded gases within the paint. I found that the gas could be expelled by simple stirring after which the weight per gallon is increased to approximately the same as the roller milled product.

Since the residual air held in the paint in no manner harms the paint, and at the same time causes an increase in the bulk, as well as an immovement in the hiding power, I prefer not to remove it.

- EXAMPLE II The equipment used in this example consisted of a small air operated paint spray gun and a small open can. A colored paint having the following composition was made:

Pigments 1 Grams Blue lead 838 Lead chromate 468 Zinc oxide 156 Vehicles Raw linseed oil 82B Lead napthenate drier 18 Cobalt napthenate drier 4 The pigments and vehicle were placed in the chamber of the gun and thoroughly mixed tocether.

lbs. per sq. inch against the inside wall of the receiving vessel, where the paint coalesced and ran down into the vessel.

A small portion of the paint was set aside as a sample and the remainder was returned to the chamber of the gun where it was sprayed a second time. The procedure was repeated until by test, the. degree of dispersion of the pigment in the vehicle approached that of a well dispersed roller milled product.

The resulting paints were tested for texture and degree of dispersion by the method used in Example I and the results are given below:

In this example the rough suspension was drawn upward from the chamber of the gun through a tube, due to the suction created by the jet of gases issuing from the nozzle of the gun. where it was contacted by a narrow pencil of The rough suspension was then sprayed with the gun operating at an air pressure of 55 stream of cases as an aeriform suspension. While in this state the portion of pigment particles which became disintegrated were completely surrounded by a fine mist of oil which contacted their surface to envelop them with a film of oil and completely wet them. Upon striking the surface of the container the suspension broke, the gas escaped, and the wetted pigment and oil coalesced into paint. In this case the jet of air was not of sufficient strength to completely disintegrate all of the pigment agglomerates, in a single pass through the gun, and it required four passes to obtain a high quality product. However, there are many obvious changes that could be made to improve the procedure, whichare within the scope of the invention.

In the preferred embodiment of my process there are many advantages not previously combined in the known methods of paint grinding.

Of particular importance is the emcient use of energy or power. Unlike all other known methods of applying power to the grinding of paint, my process utilizes small, light weight, easily controlled equipment, which applies a full and constant flow of energy to a definite and small amount of material in a given time, under conditions which are ideally suited to the disintegrating, wetting, and dispersing of pigments in vehicles and where drag or frictional load is cut to a minimum.

It should be stated that while in other known processes the vehicle surrounds the pigment in a continuous phase, the present process breaks up the vehicle into a discontinuous phase and while in this condition the vehicle, comes into contact with thcfinely divided suspended pigment particle and surrounds them in an envelope. of the vehicle. Furthermore, the products of my process have certain definite advantages not possessed by the products of other processes. Of particular importance is the constancy of quality in the finished paint, which is obtained through the elimination of the personal factor and other inherent faults in the regulating or setting of the conventional mills.

In the operation of my impact disperser method it is virtually impossible to produce anything but a well dispersed p oduct because the conditions under which a given paint is to be manufactured are fixed in advance and cannot be materially altered while the apparatus is in motion.

In the examples given to illustrate the practice of my invention, the gasused as a dispersing agent also served as a cooling agent, and in each instance the gas used was air. However, it is understood that other gases could be used and should be selected to suit certain requirements such as the case where explosive mixtures are being ground.

In contradiction to other known paint grinding methods, my process can be adapted forthe manufacture of all types of paint in a single step through the use of a single machine.

While I have disclosed a method of grinding paint it is to be understood that the same is equally applicable to the manufacture of printer's ink and lacquers,'and whenever the term paint is used 'either in the specification or the claims it is my intention to include printing inks and lacquers therewith,

Having thus disclosed my invention, what I claim as new and desire to secure by Letters Patent is:

1. The method of manufacturing a paint product in which the pigment is in a very finely divided state in a liquid paint vehicle, which comprises mixing pigment with the vehicle forming a rough or partially wetted suspension of the pigment in the vehicle, then subjecting the rough or partially wetted suspension t an impacting force in the presence of a gaseous medium to form a gaseous suspension of finely sub-divided pigment particles in an envelope of vehicle, then finally breaking the latter suspension by removal of the gaseous medium whereby the pigment and vehicle coalesce.

2. The method of manufacturing a paint product in which the pigment is in a very finely divided state in a liquid paint vehicle, which comprises mixing pigment with the vehicle forming a rough or partially wetted suspension of the pigment in the vehicle, then subjecting the rough or partially wetted suspension to an impacting gaseous stream subdividing the pigment and vehicle to produce a completely wetted product comprising a gaseous suspension of finely divided pigment particles in an envelope 0! vehicle, then finally breaking the latter suspension by removal of the gas forming the stream whereby the pigment and vehicle coalesce.

3. The method of manufacturing a paint product in which the pigment is in a very finely divided state in a liquid paint vehicle, which comprises mixing pigment with the vehicle forming a rough or partially wetted suspension of the igment in the vehicle, then subjecting the rough or partially wetted suspension to the action of the hammers of a hammer mill in the presence of a gaseous medium to subdivide the pigment and vehicle to produce a completely wetted product comprising a gaseous suspension of finely subdivided pigment particles in an envelope of vehicle, then finally breaking the latter suspension against the inner surface of the hammer mill to remove the gaseous medium whereby the pigment and vehicle coalesce.

FORREST L. 'I'URBE'IT. 

